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ITTL the General Staff wanted a heavier gun capable of bunker busting as well as firing H.E. for infantry support. Therefore instead of one main turret mounting a 3pdr (47mm) gun and four subsidiary turrets mounting 0.303 Vickers machine guns the tank was fitted with a single turret mounting one 3-inch (76mm gun) with a coaxial Vickers MG with a 3 man crew (commander, gunner and driver) plus a driver and radio operator in the hull. The latter also operated the hull mounted Vickers MG. The gun was an adaptation of the 3-inch anti-aircraft gun used by the British Army in large numbers in World War One.
The British Army later adapted its 3.7-inch AA gun into the 32-pdr anti-tank gun. Would the same be possible with the 3-inch? AFAIK there were hundreds of them in storage in the 1920s.